In oil well drilling and completion operations, a bore hole tool assembly in the form of a tubular drill string is used. These drill strings can consist of various types and sizes of tubular components like drill pipes, heavy weight drill pipe, drill collars, liner, casing and tubing. These components can be used in various configurations.
These drill strings are subjected to mechanical tension and compression due to the weight of the drill string and friction against the well bore wall. A bearing assembly is required, if a portion of the drill string is to be independently rotated from the remaining part of the drill string. Such bearing assembly is subjected to the same tensile and compressive loading as the surrounding tubular members in the drill string.
In this type of use, the bearing assembly generally is provided in the annular space that is available between a tubular end of one drill string component, functioning as the mandrel, and a tubular end of another drill string component, functioning as the housing.
The cross sections of the mandrel and that of the housing are often constrained as a consequence of practical requirements. For instance, the maximum cross section of tubulars for use in a well bore is limited by the need of an annular space between the well bore and the tubulars for allowing sufficient flow of a drilling fluid containing drilled cuttings. The minimum cross section may be constrained by the demand for allowing sufficient flow of the drilling fluid or for enabling passage of objects of a given size such as cementation plugs or wire-line deployed tools. In addition, a minimum wall thickness of mandrel and housing are required for providing sufficient strength.
These requirements limit the space available for mounting the bearing assembly in the annular space. A single bearing stage that fits in the available space may have insufficient strength to bear the load that the bore hole tool assembly is subjected to. Therefore, it has been proposed to use a bearing system having a plurality of bearing stages arranged to transfer part of the axial load parallel to each other.
In practice, the respective bearing stages are arranged in the annular space with an axial displacement relative to each other, such that they act parallel to each other in transferring the axial load from the housing to the mandrel.
It has been found that, where each bearing stage of the bearing system can transfer a certain maximum static load before failure, the bearing series already fails under a static load that is significantly lower than the mentioned maximum load per bearing stage times the number of bearing stages in the series. Moreover, under rotary conditions the endurance life of the system is significantly less that would be expected from the specified endurance life of the individual bearings.
It is thus an object of the invention to increase the maximum allowable load for the bore hole tool assembly, and in particular for the bearing system.
It is another object of the invention to provide a bore hole tool assembly, and a bearing system for such a bore hole tool assembly, having a longer operational lifetime.